In known thermal printers, an array of heater elements on a substrate is held in contact with thermally sensitive paper. The individual heater elements are then selectively energized to form a desired array of dots on the thermally sensitive paper. Thereafter the paper is shifted and another desired array of dots is created on the paper. The process is continued and the dots on the thermal paper form individual letters and numbers, each individual character consisting of a group of dots.
Printer heads which use thick or thin film resistors on a ceramic substrate as heating elements for marking thermally sensitive paper are known in the art. The limitiations of such thermal printer heads are the need for an excessive number of lead out connections from the thermal printer head and/or the requirement for the use of isolation devices, such as diodes, either on the thermal printer head itself or in the peripheral circuitry. Isolation devices are required to prevent the energizing of undesired heating elements when individual heating elements are connected in a matrix in order to reduce the complexity of circuits that select which heating elements are to be energized.
Some thermal printer heads have been fabricated using mesa transistors instead of thick film resistors as the heating elements, and thus the semiconductor element itself serves as the heating device. These printer heads do eliminate the need for additional seimconductor devices for isolation, but the expense of mounting an array of individual semiconductor devices on a printer head far exceeds the expense of formulating a network of thick or thin film resistors on a ceramic substrate.
Resistors having a voltage-current switching characteristic are common and they are used either as voltage transient suppressors or as replacements for semiconductor switches. These resistors have not, however, been used as both heating and switching elements.